Hypoxia inducible factor-1 (HIF-1) plays a central role in the development and progression of tumors. While not wishing to be bound by theory, it appears this is because HIF-1 controls the expression of more than 40 target genes whose protein products play crucial roles in allowing the survival of cells under adverse environmental conditions and in response to radiation or chemotherapy. These include the gene encoding VEGF, which is required for tumor angiogenesis, insulin-like growth factor 2 (IGF2), which promotes tumor cell survival, and glucose transporters 1 and 3, and glycolytic enzymes such as aldolase A and C, hexokinase 1 and 3, lactate dehydrogenase A and PGK.
HIF-1α is a subunit of HIF-1. HIF-1α protein is found in a wide variety of human primary tumors but only at very low levels in normal tissue. The importance of HIF-1α to cancer is demonstrated by the high incidence of tumors such as renal cell carcinoma, pheochromocytoma and hemingioblastoma of the central nervous system in individuals with loss of function of both alleles of the VHL gene leading to elevated HIF-1α levels. In addition, most cases of sporadic renal cell carcinoma are associated with an early loss of function of the VHL gene and increased HIF-1α levels. Reintroduction of the intact VHL gene into cells derived from renal carcinomas restores HIF-1α to normoxic levels and decreases tumorigenicity. HIF-1α levels are also increased in cancer cells with mutant or deleted PTEN.
Many human tumors have been shown to overexpress HIF-1α protein as a result of intratumoral hypoxia and genetic alterations affecting key oncogenes and tumor suppressor genes. In addition, over-expression of HIF-1α correlates with treatment failure and mortality. However, loss of HIF-1 activity has dramatic negative effects on tumor growth, vascularization and energy metabolism in xenograft assays. Therefore inhibition of HIF-1 represents a promising new approach to cancer therapy since its inhibition may lead to the selective killing of tumor cells over normal cells.
The HIF-1α inhibitor PX-478 (S-2-amino-3-[4′-N,N,-bis(2-chloroethyl)amino]phenyl propionic acid N-oxide dihydrochloride) inhibits growth of hypoxic tumor cells in vitro. PX-478 inhibits HIF-1α protein, leading to decreased HIF-1 transactivation and expression of the downstream target gene VEGF. PX-478 also decreases HIF-1α in vivo at a non-toxic dose. Interestingly, this inhibition has been shown to occur independently of the VHL pathway, the most well-studied mechanism for controlling HIF-1α stabilization.
Many other factors have been shown to affect HIF-1α protein, including the P53 tumor suppressor pathway as well as oncogenes signalling through the P13K and MAPK pathways. Several recent studies have also reported indirect inhibition of the HIF-1 pathway in a VHL independent manner. These include inhibition of P13K using LY294002, inhibition of the molecular chaperone HSP90 using geldanaycin, and inhibition of redox signalling by PX-12 (1-methylpropyl 2-imidazolyl disulfide) and pleurotin. Indeed, thioredoxin reductase activity was shown in this study to be significantly decreased at concentrations of PX-478 which correlate well with HIF-1α inhibition.
A recent study has shown that Trx-1 binds to, and inhibits, the tumor-suppressor protein PTEN leading to activation of the P13K pathway through AKT. In light of the findings that the P13K/AKT pathway is involved in the stabilization and activation of HIF and that the P13K inhibitor LY294002 also decreases HIF-1α protein in a VHL independent manner, it is possible Trx-1 may affect HIF-1α through this pathway. Recent studies suggest that this is cell-type dependent and, when observed, lies downstream of HIF activation or in a parallel pathway.
One goal of targeted therapies for disease treatment, such as the HIF-1α inhibitor PX478 in treating cancer, is to be able to select patients that are most likely to respond to the drug. In the case of cancer, while immunohistochemical techniques verifying the upregulation of HIF-1α in the target tumor is the gold standard, often invasive procedures such as tumor biopsies are not possible and the tumor tissue required for such tests are not available. Non-invasive techniques have been explored to evaluate the effect of PX478 on HIF-1α levels in the clinical setting, including the use of Dynamic Contract Enhanced-magnetic resonance imaging (DCE-MRI) and diffusion weighted (DW) MRI to evaluate tumor vascular permeability. It is possible that even though a tumor may express some level of HIF-1α, the protein some patients express may not be responsive to a synthetic inhibitor such as PX-478. In order to provide effective treatment to an individual, it would be helpful to identify those individuals who will be responsive or susceptible to an HIF inhibitor (e.g., PX-12, PX-478, 2-ME2). It would be useful to be able to identify patients with hypoxic tumors and corresponding increased HIF-1α levels, and to be able to evaluate the HIF-1α inhibition following therapy.